In energy conversion and power transmission it is often necessary or desirable to convert the pressure ratio of pressure oscillations of one fluid circuit to a higher or lower value in a coupled fluid. For example, if the pressure ratio of a first fluid circuit equals 2.0, the pressure ratio of the coupled fluid may need to be increased to 8.0 or reduced to 0.5 for meeting a particular requirement. The virtues of pressure ratio amplification may be illustrated by considering first an air compressor and then a hydraulic pump application.
It should be noted that there are several ways to describe a situation where a working fluid operates between two extremes in pressure (P.sub.min and P.sub.max). For example, one can use P.sub.min and P.sub.max, or alternately the mean pressure ((P.sub.min +P.sub.max)/2) and the pressure amplitude ((P.sub.max -P.sub.min)/2). The remaining discussion in this application will be in terms of a third pair of variables, the pressure ratio (P.sub.max /P.sub.min) and the mean pressure.
A typical crankshaft-driven reciprocating air compressor compresses air of atmospheric pressure (14.7 psia) to 100 psig (114.7 psia) discharge pressure, thereby operating at a pressure ratio of (114.7/14.7) or 7.80. However, if a particular application required a discharge pressure of 300 psia, this compressor could not be used since the crank bearing and drive motor would be overloaded. Alternately, if a discharge pressure of 30 psig were required at 3 times the rated cfm of the compressor, either three identical compressors or a new compressor would be required. Thus, the compressor that had been designed for operation at a pressure ratio of 7.80 is seen to be incapable of operating at a higher pressure ratio (21.41) and has insufficient capacity when the demand is at a lower pressure ratio (3.04).
The second example involves a free piston engine driving a hydraulic pump which is used to power a hydraulic motor. A typical free piston engine might employ a working gas operating between pressures of 200 and 400 psia (pressure ratio of 2.0). The engine working gas operates on a diaphragm which separates it from the pumped hydraulic fluid. To provide a long lifetime for the diaphragm, the respective pressures on both sides of the diaphragm must be equal at any instant of time. As a result, the hydraulic fluid pressure equals the working gas pressure, and the hydraulic fluid must be discharged at an outlet pressure of 400 psia and admitted at an inlet pressure of 200 psia. That is, the pump pressure chamber operates at the same pressure ratio of 2.0. Thus, if the hydraulic motor is to be matched to the pump output, the motor would properly operate with a 200 psi pressure differential across it. However, the hydraulic motor may be smaller and lighter if it operates with a supply pressure of 4000 psia and a discharge pressure of 50 psia. This would require that the pump (and also the engine) provide a pressure ratio of 4000/50=80. However, no known free piston engine is capable of operating at such a high working gas pressure ratio.
One possible way of providing the required 4000 psia pressure for the motor would be to add a pressure intensifier between the existing engine driven diaphragm pump and the hydraulic motor. The intensifier, for example, could comprise a stepped piston with mating bores where the smaller of the bores (the high pressure side) had a cross-sectional area one-tenth that of the larger diameter low pressure bore. This intensifier would increase the pump output pressure from 400 to 4000 psia, but unfortunately, it would also increase the pump inlet pressure from 200 to 2000 psia which is far from the required 50 psia. Thus, since pressure intensifiers amplify pressure, but not pressure ratio, they cannot be used for this specific application.
In the two examples described above, wherein a fluid coupled load required a pressure ratio that differed from that available from the working gas, it was nevertheless assumed that the requirement was a constant one. The actual situation is likely to be even worse, since many applications are characterized by an output pressure ratio requirement that varies over a range. For example, a pressure reservoir is usually pumped up from a low pressure to a high pressure, so that the pump load exhibits a variable pressure ratio. Such is the case when an air compressor is used to pressurize an air tank or a hydraulic pump is used either to pressurize an accumulator or to supply a hydraulic motor subject to variable torque load.
The problems discussed above have generally been accepted as inevitable, with the consequence that many sources of pressure oscillation have been ignored for wide classes of applications. For example, while free-piston Stirling engines may be attractive for a variety of reasons, their operating characteristics make them ill-suited for pumping hydraulic fluid to power a hydraulic motor. Accordingly, while not heretofore well recognized, there is presented the need for a device and method that provide pressure ratio transformation in fluid circuits. The situation is exacerbated by the additional factor that the required transformation ratio is often determined by the instantaneous load characteristics.